OPEN
SUBJECT AREAS:
TECTONICS
Recognizing detachment-mode seafloor
spreading in the deep geological past
Marco Maffione*, Antony Morris & Mark W. Anderson
PALAEOMAGNETISM
GEOLOGY
School of Geography, Earth and Environmental Sciences, Plymouth University, Drake Circus, Plymouth PL4 8AA, UK.
GEODYNAMICS
Received
22 March 2013
Accepted
10 July 2013
Published
1 August 2013
Large-offset oceanic detachment faults are a characteristic of slow- and ultraslow-spreading ridges, leading
to the formation of oceanic core complexes (OCCs) that expose upper mantle and lower crustal rocks on the
seafloor. The lithospheric extension accommodated by these structures is now recognized as a
fundamentally distinct ‘‘detachment-mode’’ of seafloor spreading compared to classical magmatic
accretion. Here we demonstrate a paleomagnetic methodology that allows unequivocal recognition of
detachment-mode seafloor spreading in ancient ophiolites and apply this to a potential Jurassic detachment
fault system in the Mirdita ophiolite (Albania). We show that footwall and hanging wall blocks either side of
an inferred detachment have significantly different magnetizations that can only be explained by relative
rotation during seafloor spreading. The style of rotation is shown to be identical to rolling hinge footwall
rotation documented recently in OCCs in the Atlantic, confirming that detachment-mode spreading
operated at least as far back as the Jurassic.
Correspondence and
requests for materials
should be addressed to
M.M. (m.maffione@
uu.nl)
* Current address:
Paleomagnetic
Laboratory Fort
Hoofddijk, Department
of Earth Sciences,
University of Utrecht,
Budapestlaan 17,
3584 CD Utrecht, The
Netherlands.
I
t is now known that large-offset detachment faulting plays a fundamental role in accommodating plate
separation at slow- and ultraslow- spreading mid-ocean ridges1–7. For example, recent work3 has concluded
that more than 50% of the Mid-Atlantic Ridge between 12.5u N and 35u N is affected by such faulting.
Recognition of the widespread occurrence and significance of these structures has led to definition of a new
‘‘Chapman’’ detachment-mode of seafloor spreading8. The uplifted footwalls of oceanic detachment faults are
known as oceanic core complexes (OCCs)2–7. They are elevated, dome-shaped massifs, typically with prominent
spreading-parallel corrugations on smoothly curved upper surfaces (Fig. 1). These surfaces represent exposed,
now-inactive low-angle fault planes that have been directly responsible for the exhumation of mantle and plutonic
lithologies onto the seafloor. Paleomagnetic analyses of OCC footwall sections sampled by scientific ocean drilling
along the Mid-Atlantic Ridge9–11 have demonstrated that this unroofing is characteristically accommodated by
tectonic rotation around ridge-parallel, shallowly plunging axes, consistent with flexural, isostatic rolling-hinge
deformation12,13.
Given the widespread occurrence of oceanic detachment faults and associated OCCs in young lithosphere
close to present day spreading axes, there is clearly a need to establish whether this mode of spreading was
also significant in the deep geological past. Exposed OCC footwall sections have only been identified and
directly sampled in young (,11 Ma) lithosphere2,5–7,9–11. Older oceanic detachment systems in in situ lithosphere are obscured beneath marine sedimentary successions, but fault systems that are geometrically
compatible with detachment-mode spreading have been imaged by seismic reflection experiments in
Cretaceous oceanic crust in the eastern Central Atlantic14. To extend this record to older geological time
periods, it is necessary to search for potential examples of detachment-mode fault systems in ancient oceanic
lithosphere preserved in ophiolites. However, recognition of these structures in ophiolites requires separation
of seafloor spreading and emplacement-related tectonic signatures, as primary seafloor morphologies are
rarely preserved in ophiolites. This can only be achieved by linking evidence for tectonic juxtaposition and
relative rotation of upper crustal and lower crustal/upper mantle sequences to kinematic constraints in an
original seafloor frame of reference.
We illustrate this approach using an example from the northern Mirdita ophiolite of Albania. This is a slice of
Jurassic (c. 165 Ma15) oceanic lithosphere, representing a remnant of the eastern branch of the slow-spreading
Tethys Ocean that was obducted during Europe-Adria convergence15,16. It consists of a partially serpentinized
lherzolitic mantle sequence (containing discrete gabbro intrusions) overlain tectonically by an upper crustal
sequence of sheeted dikes and lavas (Fig. 2)17. The contact between mantle and upper crustal rocks in the region of
the Puka Massif is locally marked by mylonitic shear zones in amphibole-bearing peridotites17,18. These field
relationships17,19 may result from either (a) detachment-mode spreading and associated mantle exhumation (as
SCIENTIFIC REPORTS | 3 : 2336 | DOI: 10.1038/srep02336
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Figure 1 | The ‘‘Chapman model’’8 of lithospheric accretion and oceanic core complex formation dominated by detachment faulting. In this newly
recognized mode of seafloor spreading, plate separation is accommodated by displacement on large-slip detachment faults, leading to rotation of
fault footwalls, exhumation of mantle lithosphere, and the formation of oceanic core complexes.
seen in modern OCCs1–8) or (b) local tectonic excision of the lower
crustal section during ophiolite obduction or post-emplacement
deformation. These alternatives may be resolved by detection of
OCC-type relative rotation of detachment footwall and hanging wall
sections around a ridge-parallel axis, as seen in in situ OCCs in
Atlantic lithosphere. Such rotation cannot be detected from field
observations alone as paleohorizontal indicators are typically absent
in ophiolitic rocks, but can be quantified using magnetic remanence
vectors from footwall and hanging wall blocks. Here we combine
paleomagnetic detection of relative rotation with field structural data
from the Mirdita sheeted dike complex to show that the geometry
and kinematics of rotation are consistent with OCC-style flexural,
isostatic rolling-hinge12,13 deformation. This extends the record of
‘‘Chapman’’8 detachment-mode spreading back to the Jurassic, to
the age limit of in situ oceanic lithosphere on Earth.
Results
We sampled gabbroic intrusions and variably serpentinized host
peridotites in the upper mantle footwall sequence of the inferred
Mirdita detachment fault around the Puka Massif, and basaltic
sheeted dikes, pillow lavas and sheet flows and cross-cutting dikes
in the upper crustal hanging wall sequence (Fig. 2). Demagnetization
experiments were used to isolate magnetic remanences of the
sampled specimens and rock magnetic techniques used to determine
the nature of the main magnetic carriers (see Methods). Stable magnetization directions were obtained at 25 upper mantle and upper
crustal sites. Typical examples of demagnetization behaviour for each
lithology are illustrated in Figure 3a–e. Rock magnetic experiments
and demagnetization characteristics indicate that multidomain titanomagnetite and titanomaghemite are the main magnetic carriers in
the upper crustal lithologies, whereas single-domain to pseudosingle-domain magnetite and low-titanium titanomagnetite are
responsible for the magnetization of gabbros and peridotites. Mean
in situ directions of magnetization for mantle peridotites (PP) and
mantle-hosted gabbro intrusions (GA) of the Puka Massif and for
upper crustal sheeted dikes (SD) and lavas/discrete dikes (LD) are
shown in Figure 3f. These directions are significantly different from
the present-day magnetic field direction at the sampling locality (D
5 0u, I 5 61u), and are interpreted as ancient remanences.
SCIENTIFIC REPORTS | 3 : 2336 | DOI: 10.1038/srep02336
The timing of remanence acquisition is difficult to constrain using
paleomagnetic tilt tests in ophiolitic terranes due to the lack of
unequivocal paleohorizontal surfaces in extrusive rocks and the
potential for components of tilting around dike-normal axes in
sheeted dikes20. However, remanence acquisition during or shortly
after formation by seafloor spreading in the upper crustal sites is
supported by: (i) the presence of normal (10 sites) and reversed (4
sites) polarities of remanence, consistent with rapid polarity changes
in the Jurassic21 and inconsistent with pervasive later remagnetization; and (ii) mean inclinations of the upper crustal sites (in both in
situ and tilt corrected coordinates) consistent with that expected
from the Jurassic (170 Ma) European paleomagnetic pole22, assuming the ophiolite is associated with the European, overriding plate of
the Tethys realm during intraoceanic subduction.
The gabbros within the mantle sequence in the footwall block
carry a high temperature, single component remanence of reversed
polarity that unblocks close to the magnetite Curie temperature
(580uC, with only limited unblocking below 525uC; Fig. 3e). These
discrete, high unblocking temperature spectra are consistent with
remanence characteristics of gabbros recovered by scientific ocean
drilling10,23,24 and compatible with a primary thermoremanence
carried by nearly pure, single-domain magnetite. Also, the lack of
alteration of the gabbros precludes acquisition of significant postemplacement chemical remanence by these rocks.
The footwall gabbros (GA in Fig. 3f) and hanging wall upper
crustal section (SD and LD in Fig. 3f) record statistically different
magnetization directions. The NNE magnetization of the hanging
wall sequence reflects a 45u clockwise regional rotation of the
Albanides during the Tertiary25, whereas ENE magnetizations of
the footwall gabbros require additional substantial rotation relative
to the hanging wall. In contrast to the reversely magnetized footwall
gabbros, the host mantle carries a normal polarity remanence
(Fig. 3d) with a significantly different direction to that of the gabbros
(Fig. 3f). Hence, remanences in different footwall units within the
Puka Massif were acquired at different times. The relative ages of
magnetization are clearly constrained by detailed sampling of a ,
2 m interval across a sharp intrusive contact (sites PU05/06 in Fig. 2),
where significantly different magnetization directions are observed
right up to the contact. The remanence of the host mantle peridotites
must postdate gabbro emplacement, as any remanence acquired
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20° 00’
0
40
Mirdita
ophiolite
400
600
800
80
0
600
1000
0
60
PU05/06
80
0
00
42° 00’
14
0
120
0
80
Puka Massif
1400
Detachment footwall
600
600
0
100
Spac
0
80
0
120
400
5 km
Mantle sequence
(peridotite/gabbro)
60
0
Detachment hanging wall
Reps
Crustal sequence
(undifferentiated)
Inferred detachment fault
Fault
Amphibolite
Sampling site
Figure 2 | Simplified geological map of the study area in the northern
Mirdita ophiolite. The Mirdita detachment fault19 separates a hanging wall
sequence consisting of sheeted dikes, lavas and discrete dikes from a
footwall sequence of partially serpentinized peridotites containing discrete
gabbro intrusions (map adapted from the 15200.000 Geological Map of
Albania34).
prior to intrusion would be thermally overprinted immediately adjacent to the contact producing a secondary magnetization identical in
direction to that of the later gabbro. Remagnetization of the host
peridotites after gabbro intrusion is most likely due to production
of secondary magnetite during partial serpentinization26. The mean
direction of magnetization of the footwall peridotites is indistinguishable from that of the upper crustal rocks of the hanging wall
(PP and LD 1 SD, respectively; Fig. 3f), suggesting remanence
acquisition when blocks either side of the inferred detachment fault
were in their current relative positions. The common paleomagnetic
direction shared by peridotites and upper crustal rocks at the Puka
Massif, combined with the statistically different, demonstrably older
magnetization of the footwall gabbros, indicates that magnetization
of the peridotites occurred after detachment-related rotation, but
broadly coeval with magnetization of the unrotated upper crustal
rocks in the hanging wall. Active detachment faulting normally
occurs within a few kilometers of the ridge axial valley3 during
short-lived (1–3 Ma4,5) periods of detachment-dominated spreading.
Since major serpentinization (with associated magnetization acquisition) takes place in the footwall peridotites during faulting activity27,
SCIENTIFIC REPORTS | 3 : 2336 | DOI: 10.1038/srep02336
the age of magnetization of the peridotites and upper crustal rocks is
therefore likely to be very similar (within , 3 Ma). Similar relationships between the age of remanence in footwall gabbros and their
host peridotites have been established at OCCs associated with the
Fifteen-Twenty Fracture Zone in the Atlantic9, sampled during
Ocean Drilling Program Leg 209, although early stage (pre-rotation)
magnetization has also been inferred for some peridotites11. A common paleomagnetic direction between footwall and hanging wall
units also excludes their relative rotation during ophiolite emplacement and Alpine tectonics, further restricting footwall rotation to
detachment activity during Jurassic spreading.
The statistically different mean remanence directions of footwall
gabbros and hanging wall upper crustal rocks must result from relative net tectonic rotation across the intervening detachment fault.
Infinite solutions exist for the orientation of the axis and the magnitude of this rotation, defining a locus that forms the great circle
bisectrix of the footwall and hanging-wall paleomagnetic vectors
(Fig. 4a). Rotation angle is highly sensitive to choice of rotation axis
along this locus9,11. This choice may be limited by making the geologically reasonable assumption that footwall rotation occurred
around a ridge-parallel axis9,10. The paleo-ridge orientation in ophiolite terranes may be determined reliably from regionally consistent
orientations of dikes in the sheeted dike complex (which are considered to be emplaced vertically along axis during seafloor spreading17,28). Measurement of mean dike orientations at 56 locations
within the 10 km long section of sheeted dike complex between
the town of Reps and Spac (Fig. 2) indicates a consistent pattern of
sub-vertical dikes with an average NNE-SSW trend (026u/82u, a95 5
5.3u). This agrees with regional dike trends in the northern Mirdita
ophiolite17,18. A single net tectonic solution for the footwall rotation
(axis 5 029u/11u, magnitude 5 46u counterclockwise) is then provided by the intersection of the mean dike orientation and the great
circle bisectrix of footwall and hanging wall mean remanence directions (white star in Fig. 4a).
Confidence limits (a95 values) on both remanence directions and
the mean dike orientation result in uncertainties in these rotation
parameters, generating a wasp-waisted envelope of potential great
circle bisectrices and a narrow envelope of permissible dike orientations (shaded regions of Fig. 4b). To model the effect of these uncertainties, we conducted a Monte Carlo simulation, selecting 1,000
pairs of points randomly distributed within the a95 cones of confidence of the footwall and hanging wall magnetization vectors to generate 1,000 statistically acceptable great circle bisectrices. These were
combined with 1,000 dike orientations randomly selected within the
a95 confidence limits on the mean dike orientation, yielding 1,000
estimates of the mean rotation axis and magnitude. Figure 4c illustrates the resulting distributions of rotation axes and angles. The
orientation of the maximum density of the cluster of rotation axes
is 028u/16u, and is taken as the best estimate for the relative rotation
axis in present day geographic coordinates together with the median
counterclockwise rotation angle of 44.5u. This provides a minimum
estimate of the total rotation because of the potential for early rotation of the footwall at temperatures above the magnetite Curie temperature (580uC) or prior to gabbro intrusion.
The observed strike of the sheeted dikes is orthogonal to the
regional emplacement WNW-ESE direction of the Mirdita ophiolite18. The sub-vertical orientation of the sheeted dikes therefore
suggests that there has been only limited net tilting of the Mirdita
upper crustal section during ophiolite emplacement17. Hence, the
present day subhorizontal relative rotation axis was likely to have
originally been subhorizontal prior to emplacement.
Discussion
Our modelled solution for footwall rotation accommodated by the
Mirdita detachment (Fig. 4c) is geometrically consistent with mantle
unroofing and denudation via a flexural, rolling-hinge mechanism12,13,29
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a
b
N Up
c
N Up
d
N Up
NRM
N Up
NRM
NRM
Peridotite
PM0304b
400°C
10mT
4mT
15mT
250°C
E
49mT
E
400°C
Pillow lava
Pu3902b
Sheeted dike
RE0207C
S Down
e
N Up
E
30mT
Dolerite dike
PU2020iA
S Down
NRM
S Down
S Down
f
N
Gabbro
PU0607
W
E
585°C
575°C
Unit
N
Dec
Inc
LD
SD
PP
GA
8
31.8
6
36.2
8
27.4
3 243.4
50.1
47.3
44.0
-32.0
k
α95
50.0 7.9
77.1 7.7
43.8 8.5
41.3 14.5
PP
SD
NRM
LD
525°C
GA (antipode)
S Down
E
Figure 3 | Summary of paleomagnetic results from the Puka Massif and surrounding region. (a–e) Representative orthogonal vector plots of thermal
and alternating field (AF) demagnetization for each lithological group (in situ coordinates). Solid/open circles represent the projection of the remanence
vector onto the horizontal/vertical plane, respectively; NRM, natural remanent magnetization; specified demagnetization steps are in degrees Celsius (uC)
or millitesla (mT). (f) Equal area stereographic projection of the mean directions of pillow lavas and dikes (PD), peridotites (PP), sheeted dikes (SD), and
gabbros (GA), with associated a95 cones of confidence (in situ coordinates). Note that the antipode of the reversed magnetization of the gabbros is shown
to allow direct comparison with other units; black star indicates the present day geocentric axial dipole (GAD) field direction for the Puka region;
grey triangle indicates the expected direction for the 170 Ma European paleopole22. Inset table: N 5 number of sampling sites; Dec 5 declination (u) of the
mean remanence direction; Inc 5 inclination (u) of the mean remanence direction, k 5 Fisher32 precision parameter; a95 5 semi-angle of the 95% cone of
confidence (u) around the mean direction.
during detachment-mode seafloor spreading. It may be compared
directly to solutions derived from core samples collected from
young OCCs along the Mid-Atlantic Ridge. Paleomagnetic analyses at Atlantis Massif OCC (Integrated Ocean Drilling Program
Expedition 304/3057) indicate a minimum 46u footwall rotation10,
making the assumption that the footwall rotation axis was horizontal (Fig. 4d). At the 15u459 N OCC, direct constraints on
potential rotation axes have been derived from structural analysis
of fully oriented core samples collected using a sea-bed rock drill
system5, demonstrating a 64u footwall rotation around a sub-horizontal axis11 (Fig. 4e). In both of these examples of well-constrained, young OCCs, footwall rotation axes were found to be
parallel to the local orientation of the Mid-Atlantic Ridge (consistent with ridge-orthogonal slip directions inferred from corrugations on exposed detachment surfaces2,5), supporting the key
assumption used in our analysis.
SCIENTIFIC REPORTS | 3 : 2336 | DOI: 10.1038/srep02336
Recognition of the importance of Chapman detachment-mode
spreading8 is based largely on bathymetric and geophysical observations in young lithosphere formed at slow- to ultraslow-spreading
rate ridges2–4. Our understanding of the tectonic processes and conditions of formation of these young structures has benefitted from
direct sampling and borehole logging of OCC footwall sequences by
scientific ocean drilling2,5,7,10,11,30. Seismic reflection profiling in older
crust in the eastern Central Atlantic14 provides evidence for
Cretaceous fault structures buried by later sedimentary sequences
that have geometries compatible with detachment-mode spreading.
However, these data provide only indirect evidence for footwall rotation in inferred ancient OCCs. The striking similarity (Fig. 4c–e)
between the rotation analysis of the Mirdita detachment fault system
presented here and well-constrained examples in the Atlantic9–11 now
provides compelling evidence that rolling hinge rotation during
detachment-mode of spreading8 also operated in the deep geological
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N
N
N
029°/11°
46° CCW
Rotation axis
Envelope of
permissible
bisectrix
orientations
HMV
Mean
dike pole
Rotation axis 028°/16°
(maximum density)
FMV
Median = 44.5°
200
Envelope
of permissible
dike orientations
Great circle
bisectrix
b
a
N
100
0
c
N
46° CCW
GAD
Frequency
Mean dike
orientation
300
30 60 90 120
Counterclockwise
rotation (˚)
64° CCW
FMV
GAD
FMV
N
Mean structural
no-slip vector
FMV
Atlantis Massif OCC
d
15°45’N OCC
GAD
e
Figure 4 | Equal area stereographic projections illustrating the geometry of rotation solutions for footwall rotation in the Mirdita ophiolite and young
OCCs on the Mid-Atlantic Ridge. (a) Footwall rotation axes lie on the great circle bisectrix of the footwall and hanging-wall magnetization vectors (FMV
and HMV, respectively). Assuming a ridge-parallel rotation axis, a single solution is found from the intersection of the mean dike orientation with this
bisectrix. (b) Envelopes of bisectrix and dike orientations (grey shaded areas) resulting from 95% confidence limits on remanence vectors and the mean
dike orientation. Permissible rotation solutions lie within the region of overlap of these envelopes. (c) Contouring of 1000 permissible rotation axes
resulting from Monte Carlo analysis (combining 1,000 pairs of FMVs and HMVs randomly distributed within their respective a95 cones of confidence
with 1,000 dike orientations randomly selected within the a95 confidence limit on the mean dike orientation). Results indicates permissible rotation axes
plunging shallowly to the NNE (maximum density of the contours), with a median rotation of 44.5u CCW (inset). (d) Assuming simple tilting, the
preferred solution (white star) of footwall rotation at Atlantis Massif oceanic core complex10 indicates 46u of counterclockwise rotation around a ridgeparallel axis. The gray area represents the envelope of permissible bisectrix orientations. (e) At the 15u459 N oceanic core complex11, structural data (mean
no-slip vector 5 vector perpendicular to the slip direction) on oriented cores provide an independent control on permissible rotation axes. The gray
shaded area is the envelope of permissible bisectrix orientations. Solutions that satisfy both paleomagnetic and structural constraints lie within the dark
gray shaded region (inset), indicating a mean rotation of 64u around a sub-horizontal, ridge-parallel axis11 (white star).
past and has been a characteristic of slow-spreading systems since at
least the Jurassic.
Methods
Cores were drilled in the field using a water-cooled petrol-powered portable drill, and
oriented with both sun and magnetic compasses. Natural remanent magnetizations
were measured with an AGICO JR-6A spinner magnetometer and a 2G superconducting cryogenic magnetometer. Stepwise alternating field (AF) demagnetization was
performed using either an AGICO LDA-3 instrument or the in-line AF demagnetizing coils on the cryogenic. Stepwise thermal demagnetization was conducted using
Magnetic Measurements Ltd MMTD1 and Pyrox furnaces. Demagnetization data
were subject to principal component analysis using the AGICO Remasoft package31
and site mean directions calculated using Fisher statistics32.
The thermal variation of low-field magnetic susceptibility was measured in an
argon atmosphere using an AGICO KLY-3/CS-3 apparatus. First-order reversal
curves33 were measured on one specimen per site using a Princeton Micromag
vibrating sample magnetometer (VSM) in order to characterize the coercivity and
grain size distribution of ferromagnetic minerals within samples.
SCIENTIFIC REPORTS | 3 : 2336 | DOI: 10.1038/srep02336
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Acknowledgments
This work was funded by an EU FP7 Marie Curie Intra-European Fellowship to M.M. We
are grateful to Prof Kujtim Onuzi and Prof Ibrahim Milushi (Institute of Geosciences,
Polytechnic University of Tirana, Albania) for logistical support, and Endri Leka and Gezim
Calldani for help during sampling.
Author contributions
M.M., A.M. and M.A. wrote the main manuscript text. M.M. and A.M. prepared the figures.
A.M. performed the Monte Carlo analysis and M.M. carried out the paleomagnetic
analyses.
Additional information
Competing financial interests: The authors declare no competing financial interests.
How to cite this article: Maffione, M., Morris, A. & Anderson, M.W. Recognizing
detachment-mode seafloor spreading in the deep geological past. Sci. Rep. 3, 2336;
DOI:10.1038/srep02336 (2013).
This work is licensed under a Creative Commons AttributionNonCommercial-NoDerivs 3.0 Unported license. To view a copy of this license,
visit http://creativecommons.org/licenses/by-nc-nd/3.0
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